Such methods are used, for example, in cement plants or plants for producing propane as well as in further plants from the minerals industry field. The exhaust gases leaving the preheater conventionally have a temperature of approximately from 300 to 400° C. However, the dust filters arranged downstream typically operate only in a lower temperature range, for example below 260° C. or even below 200° C. It is further desirable to use the heat of the exhaust gas as far as possible. Operations are therefore carried out as far as possible in a so-called interconnected operation, in which the hot exhaust gas from the preheater is used in a comminuting, pelletizing and/or drying device, in particular a combined grinding and drying system. The exhaust gas thereby conventionally cools to a temperature of approximately from 90 to 120° C. In the operating phases in which interconnected operation is not possible, the kiln is operated in so-called direct operation, in which the exhaust gas is cooled to a temperature suitable for the dust filter in a cooling device.
In the catalyst arranged downstream, pollutants, for example nitrogen oxide, are reduced. For operation of the catalysts, a so-called tail-end configuration, in which the catalyst is arranged after the last dust filter, has can be used, in which the action of the catalyst is not impaired or is only slightly impaired by dust particles contained in the exhaust gas. However, the conventional catalysts used at present require exhaust gas temperatures of approximately 250° C., which thus require the temperature of the exhaust gases to be raised downstream of the dust filter. It is known to raise the temperature of the dedusted exhaust gas at least partly by means of the waste heat from the cooling zone. By means of temperature control, the temperature of the exhaust gases flowing through the catalyst is adjusted to a predetermined value of, for example, approximately 260°, irrespective of whether the kiln is in direct or interconnected operation.
Catalysts are used in exhaust gas cleaning for reducing nitrogen- or carbon-containing compounds, such as NOx, NH3, CO, CxHy, as well as dioxins and furans. Apart from the exhaust gas temperature, which is between 150 and 600° C. depending on the type of catalyst and the compounds to be reduced, important factors for efficient catalytic reduction are catalyst poisons, which can reduce the efficiency of the catalyst by chemical reactions, and particles or molecules which can reduce the effective surface area by blocking the pores. The reduction in catalytic activity can also be influenced, for example, directly by the catalyst by the catalytic oxidation of sulfur dioxide contained in the flue gas to sulfur trioxide and subsequent reaction with ammonia to ammonium bisulfate. The influence of the pollutants and the operating temperature of the catalyst are directly related. The formation of ammonium bisulfate is thus critical in particular at low temperatures, for example below 180° C. Mercury, for example, also adsorbs at low operating temperatures. Further pollutants which can adversely affect the activity of the catalyst and which occur in the exhaust gases of clinker production are, for example, alkalis, heavy metals or fine dusts. The negative influence of the pollutants is in some cases reversible. If the operating temperature is increased, ammonium salts that form, for example, or other adsorbed molecules can decompose. The baking out of pollutants in tail-end catalysts by a temperature increase is used, for example, in power plant technology and in waste combustion plants. However, the energy requirement increases as a result, since a high exhaust gas volume is to be heated.